Deflection Voltage Timing

Spatial separation of the two images is achieved by suitably timing the triggering of the avalanche circuits to provide dephasing of the waveforms as shown in figure 4.4. In this way the two fi’ames are symmetrically deflected by a displacement (corresponding to half the image size on the phosphor screen) off the camera axis. In order to achieve this, the two compensation deflection circuits must be triggered

independently with a temporal separation corresponding to the time required to scan the photoelectron beam from one aperture to the other by the framing deflectors. Due to the aperture separation and streak speeds usually attained at tiie framing aperture plane, this

temporal separation was expected to be ~600ps, ie. t2-ti -700ps, which was

experimentally verified from the calibrated transmission line delay unit employed.The rate of change of voltage applied to the deflection plates was adjustable using the trimmer resistive dividers described in chapter 2.8.1 and so any differences in the sensitivity of the framing and compensation plates could be compensated.

V o lta g e F ra m in g W a v e fo rm R e s u l t a n t C o m p e n s a tio n 1 W a v e fo rm F ram in g W a v e fo rm R e s u l t a n t C o m p e n s a tio n 2 W a v e fo rm T i m e

Figure 4.4. Relative timing of the deflection waveforms to produce spatial separation of the images at the phosphor screen.

4.6.2 The Experimental Set Up

A full electrical and optical outline of the experimental equipment is shown in figure 4.5. 00

%

266nm 1.06pm

100^

[ a I 100% Optical Delay ₁₀₀

_%

100%

50% Diffuser Demountable Picoframe II

MLQS Single Pulse Selected, Amplified Frequency “ Quadrupled Nd:YAG Laser

Figure 4,5 showing the experimental arrangement for the dynamic testing of the Picoframe II framing camera design. Key to figure, A; photodiode, B; electrical transmission line delay unit (0 to 64ns switchable in 250 ps intervals),C, D, E; continuously variable (0 to Ins) electrical transmission line delay, F, G, H; avalanche transistor sweep circuits.

A reproduction of a frame doublet appears as figure 4.6. The limiting dynamic spatial resolution (by eye) for both frames is ~5 Ip/mm with fi’ame and inter-fiame times measured as lOOps (FWHM) and 600ps respectively, deduced fiom the sampling scheme described (chapter 2.9.4) and the optical delay introduced respectively.

SHI

sm

a m m m %

IH

M

mm SM

'

S « .

■ttKS

5lSS-

• s %

Figure 4.6. Reproduction of a double frame UV result employing the Picoframe II framing camera, first frame right, second left.

4.7 Modifications To The Picoframe II

As previously mentioned it was realised that the spatial resolution was being degraded due mainly to the fringing fields associated with the compensation plates. It was found that the compensation deflectors could be moved closer together by thinning down the ceramic glass spacers between the earthing screen and the inner deflection plate. After careful analysis of the photoelectron trajectories within the deflectors it was realised that the situation could easily be further improved by moving the two framing apertures together so that the framing apertures did not lie on the deflector centre line, but were in fact offset by 0.7mm as indicated in figure 4.7.

Framing Deflectors

Compensation Deflectors

Figure 4.7 Framing and compensation deflector geometry of the modified Picoframe II camera design (not to scale). The effect of positioning the compensation deflectors off the framing aperture centreline is clearly indicated.

With the combination of these two techniques it was possible to reduce the

inter-aperture separation to 7mm. The system was reevaluated in static mode, showing an almost uniform spatial resolution of >10 Ip/mm for all applied dc bias voltages. Dynamic testing was carried out as before and the limiting dynamic spatial resolution was determined to be (by eye) at least 8 Ip/mm for both frames when referred to the photocathode. A reproduction of an image doublet is shown as figure 4.8

Figure 4.8. Double frame result obtained with the UV-sensitive Picoframe 11 camera, first frame right, second left.

FIRST FRAME

t

Ul

i

I oc 300 -1 200 - 100 - 6 0 0 4 0 0 5 0 0 3 0 0 200 DELAY (ps) (a)

Figure 4.9 (a) showing the temporal transmission function of the first frame.

SECOND FRAME

t

Î

300 -1 200 - 100 - 8 0 0 9 0 0 6 0 0 5 0 0 7 0 0 D ELA Y (p s ) (b)

Figure 4.9 (b) showing the temporal transmission function of the second frame.

The frame and inter-frame times of the system were evaluated (chapter 2.9.4) as 120 ps (FWHM) (üansmission functions shown as figure 4.9 (a) and (b)) and 400ps, allowing for the Frst time the generation of two high quality frames to be generated in under Ins. These frame and inter-ffame times correspond to a frame rate of almost 2.5 x 10^ frames per second. It has therefore been shown that the Picoframe U design can produce high quality frames at very high framing rates with UV illumination.

4.8 Analysis of Image Features

From inspection of the frame doublet reproduced it may be seen that the two images are of slightly different magnification in the streak (horizontal) and non streak directions. Initially it was believed that this was due to the loss of symmetry of the framing apertures about the undeflected photoelectron trajectories [3]. Flowever, when the framing and compensation deflectors were operated so that the photoelectron beam was swept across the framing apertures in the opposite direction, the magnification difference was noted to change between the two images. In fact it was found that the first of die two images displayed was effected in the same way, regardless of the scan direction, and likewise for the second image. From this it was apparent that the effect was directly related to the order in which the frames were generated. In figure 4.8 the right hand image is the first generated and this shows spatial 'stietching' in the scan direction, while the later frame has been 'squashed' in the scan direction.

This effect may be explained when consideration is given to the electron deflector transit time and the electrostatic fringing fields associated with the entrance and exit of a plane parallel plate electrostatic deflection geometry. Due to the finite photoelectron transit time tlirough the deflector structure and the rapidly changing deflection voltage, the electron will experience different fringing fields at the entry and exit of the

deflectors. The effect of this is to cause image defocussing (due to the induced dynamic electrostatic cylindrical lens) in the streak direction, which becomes more severe at higher stieak speeds and deflection angles. This effect is detailed by Kinoshita et. al. [3] for the case of a streak camera showing the different asymmetric change in focal length of the electron optical system with respect to electron beam deflection, however the framing camera poses a more complex problem. A full computer simulation of the Picoframe I and II camera variants has now been undertaken, and results show that the image magnification difference is indeed indicated by the combination of the transit time / fringing field effects of the framing and compensation deflectors after optimising the dynamic focusing of the electron-optical lens. The details of simulation are the subject of a colleague's PhD [4] and indicate that this effect is a function of aperture separation, deflection plate length, separation, electron velocity, and the applied dynamic deflection voltage gradient.

As usual, a compromise between the frame / inter-frame times required and the degree of spatial distortion acceptable must be accepted because these two aspects are interconnected when the double aperture geometry and plane plate deflection electrodes are employed. The use of distributed or travelling-wave type deflectors (chapter 7) will reduce the unwanted dynamic fringing field defocussing effects, but this considerably increases the deflection structure complexity. In practice the magnification difference is small for the '-lOOps frames obtained and this may be readily taken into account during image analysis. It should be noted that the Picoffame I camera does not suffer as much from the above effects due to the axial position of the framing aperture, and so reduced dynamic fringing fields associated with the framing and compensation deflectors when the photoelectrons strike the phosphor screen. Thus when the camera is operated in double-frame mode a compromise of the electron-optical focussing may be easily found.

4.9 X-Ray Evaluation of the Picoframe II Camera

Further evaluation of the picoframe II camera was undertaken with x-ray illumination by using the Merlin laser facility at AWE Aldermaston (chapter 3.2) in a double-pulse mode. Figure 4.10 shows a streak camera recording of the 532nm laser excitation beam propagated through a 200ps round-trip time delay line for calibration purposes. The streak speed employed was 1.5x10^ cm/s.

4 0 0 ps

4---—--- ► 2 0 0 ps

Figure 4.10. Temporal characteristics of the 532nm green double-pulse excitation beam showing 400ps separation, calibration sub-pulses are 200ps from main pulse.

As before, the camera was initially optimised on the UV system before the cathode was replaced with an X-ray-sensitive version (15p.m aluminium substrate and lOnm gold, chapter 3.5) and the system transported to the laser facility. Timing was achieved as per the double frame operation of the x-ray-sensitive Picoframe I camera (chapter 3.6.1). Frame doublets were achieved, and a reproduction is shown in figure 4.11

Figure 4.11.X-ray double frame result using the Picoframe II camera, first frame right, second left.

The limiting spatial resolution of both frame was deduced to be 4.5 Ip/mm at the photocathode while the frame and inter-frame times were 110 ps (FWHM) and 400 ps respectively, measured on the frequency-quadrupled Nd:YAG laser as previously described (chapter 2.9.4).

4.10 Discussion

Excellent dynamic spatial resolution has been attained with the modified

compensation deflection geometry when used in the UV-sensitive Picoframe II framing camera. It is noteworthy that the modified design relies on the triggering of the two compensation circuits to be temporally separated by -500 ps. While this temporal separation is substantially larger then the avalanche transistor circuit trigger jitter (± 20 ps) it may be expected that the accumulated trigger jitter between all three circuits would have a considerable effect upon the camera performance. However this did not seem to be major problem as the operation of this camera system was highly reproducible.

The relatively poor dynamic spatial resolution of the x-ray-sensitive Picoframe II camera system was attributed the high secondary electi’on energies emitted from the photocathode interacting with the fringing fields associated with the framing and compensation plates. The fact that images are obtained only when the elecdon beam is deflected off the tube axis means that the transit voltages of -400 V are required on the deflection plates during framing. With the increased crossover diameter resulting from the higher secondary electron energy spread of the x-ray excitation of the gold

photocatliode, the fringing field problem is further exacerbated. Jitter between the avalanche transistor circuits may be a furtlier cause of dynamic spatial resolution reduction, but as previously pointed out, this was not apparent during the evaluation of the UV-sensitive Picoframe H camera.

4.11 Conclusions

With the Picoframe II camera both UV and x-ray double frame fonnats have been demonstrated. In both cases tlie frame and inter-frame times are shorter than the

Picoframe I double frame system. The fi'ame time of just over 100 ps is better then half of the single aperture double frame system, while the inter-frame time of 400ps is some 4 times shorter. The UV-sensitive system offers good dynamic spatial resolution at 8 Ip/mm (when referred to the photocathode) for both frames, while the x-ray-sensitive version has demonstrated 4.5 Ip/mm at the photocatliode. The degradation of the resolution for the x-ray system is principally due to the higher electron energy spread which causes the photoelectron trajectories to approach both the framing and

compensation plates. An increase in the separation of both sets of deflection plates would undoubtedly increase the dynamic spatial resolution but would drastically increase the frame and inter-frarae times.

References for Chapter 4

1 W Sibbett, M R Baggs, H Niu

Proc.l5th ICI-ISP (San Diego), SPIE M2, 267, 1982

2 G G Feldman, G I Bruikhnevich, V M Zhilkina, T A Hina, V B Lebedev, V Simonov, V I Syrtsev

Proc.High Speed Photography, Videogiaphy and Photonics IV (San Diego), SPIE 693. 147,1986

3 K Kinoshita, T Kato, Y Suzuki

Proc. 14th ICHSPP (Moscow), 199,1980 4YLiu

Chapter 5

Four-Frame Operation of The UV-Sensitive Picoframe II Framing Camera

5.1 Introduction

The ability for the Picoframe range of ffarning cameras to produce multiple exposures with good resolution, fast frame and inter-frame times and no parallax is veiy attractive as a diagnostic in the studies of laser-induced plasmas. Double frame formats have been demonstrated with the Picoframe I (with visible radiation [1]) and Picoframe n type cameras in the UV and soft X-ray spectral regions (chapters 2 to 4), while the capability to exceed two frames has not been experimentally demonstrated. In order to produce more than two frames with the Picoframe I camera, multiple triangular voltage waveforms must be generated with suitable voltage amplitude, which is difficult to achieve with the pulse forming networks (PFN's) and avalanche transistor circuits presently employed. On the other hand, the use of the Picoframe II design in

conjunction with the PFN's (to generate triangular pulses) and circuits could

potentially meet the requirements for a four frame system. In this system, two frames are generated on the leading edge of the triangular waveform and two on the trailing edge. Spatial separation of the two frames in each 'doublet' may be achieved by suitably dephasing the framing and compensation deflection waveforms as previously described (chapter 4, section 6.1) while the frame doublets are separated using dedicated shift electrodes mounted within the image tube. These shift deflectors are located between the compensation deflectors and phosphor screen and are mounted orthogonally to the streak direction as shown in figure 5.1. The application of a step input allows the spatial separation of the frame doublets to be achieved as described later in this chapter.

Photographie Film Shift

Comp. 1 Deflectors Deflectors

Initial Scan Direction

Framing Comp. 2 Intensifier Cathode

Deflectors Deflectors

Figure 5.1. Schematic diagram of the Picoframe II with shift deflectors

In document Ultrafast electron optical visible / X ray sensitivity streak and framing cameras (Page 95-107)